73 lines
1.8 KiB
C
73 lines
1.8 KiB
C
#include "tommath_private.h"
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#ifdef BN_S_MP_SUB_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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int s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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{
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int olduse, res, min, max;
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/* find sizes */
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min = b->used;
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max = a->used;
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/* init result */
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if (c->alloc < max) {
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if ((res = mp_grow(c, max)) != MP_OKAY) {
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return res;
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}
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}
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olduse = c->used;
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c->used = max;
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{
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mp_digit u, *tmpa, *tmpb, *tmpc;
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int i;
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/* alias for digit pointers */
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tmpa = a->dp;
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tmpb = b->dp;
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tmpc = c->dp;
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/* set carry to zero */
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u = 0;
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for (i = 0; i < min; i++) {
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/* T[i] = A[i] - B[i] - U */
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*tmpc = (*tmpa++ - *tmpb++) - u;
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/* U = carry bit of T[i]
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* Note this saves performing an AND operation since
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* if a carry does occur it will propagate all the way to the
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* MSB. As a result a single shift is enough to get the carry
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*/
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u = *tmpc >> ((CHAR_BIT * sizeof(mp_digit)) - 1u);
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/* Clear carry from T[i] */
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*tmpc++ &= MP_MASK;
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}
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/* now copy higher words if any, e.g. if A has more digits than B */
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for (; i < max; i++) {
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/* T[i] = A[i] - U */
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*tmpc = *tmpa++ - u;
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/* U = carry bit of T[i] */
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u = *tmpc >> ((CHAR_BIT * sizeof(mp_digit)) - 1u);
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/* Clear carry from T[i] */
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*tmpc++ &= MP_MASK;
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}
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/* clear digits above used (since we may not have grown result above) */
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for (i = c->used; i < olduse; i++) {
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*tmpc++ = 0;
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}
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}
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mp_clamp(c);
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return MP_OKAY;
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}
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#endif
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